The present invention involves an improved bushing apparatus for making glass fibers and an improved method of making and using glass fiberizing bushings.
In the manufacture of continuous fibers from a molten material like molten glass, the molten material is often generated by a tank furnace and distributed to a plurality of fiberizing bushings via one or more channels and one or more bushing legs connected to the channel(s). Each bushing leg comes off the channel at about 90 degrees and contains a plurality of bushings that are spaced apart. The molten material exiting the tank furnace into the channel(s) is much hotter than desired for fiberizing and the molten material entering the bushing legs is typically hotter than desired for fiberizing, particularly when the furnace is being run close to designed capacity.
Precious metal bushings for making continuous glass fibers are well known, having been in use for more than 50 years. Many types of bushings exist for converting molten glass into continuous glass fiber and products. Typical types of bushings are shown in U.S. Pat. Nos. 3,512,948, 4,155,732, 4,272,271 and 4,285,712, the disclosures of which are hereby incorporated by reference. All the bushings shown in these patents teach the use of a perforated plate or screen, welded to the end walls and side walls at some distance above a tip plate containing hundreds of nozzles or tips where molten glass, as it emerges from the orifice of each tip, is converted to a continuous glass fiber in a known manner. These patents teach that the purpose of the screen is to condition the glass, homogenizing the chemistry and temperature, and to prevent pieces of refractory or unmelted glass from reaching the tip plate. Most, if not all, of these references teach using a uniform hole pattern with uniform hole size over the entire surface of the tip plate. Normally, the screens taught by these references improve the temperature profile of the tip plate, i.e. produce a more uniform temperature in the molten glass just above the tip plate in all directions than if the screen were not present.
These bushings work well as long as the molten material entering the bushings is fairly uniform in temperature, but often there is at least a streak of molten material in the flow that is significantly hotter than the molten material next to the walls of the channel. This hotter material has a lower viscosity than the cooler material next to the walls. When it enters the bushing, always in the first position next to the channel and sometimes in the second position of a bushing leg, it flows through holes in a conventional screen in the bushing faster than the cooler material. This causes the temperature profile of the tip plate spaced below the screen to be non-uniform. When this happens, a generally central portion of the length of the bushing tip plate runs considerably hotter than the ends. This hotter central portion can be offset to the down stream end due-to the velocity vector of the hotter stream of glass. The hot glass has a higher velocity down center of the bushing leg and down the orifices to the bushings than the colder glass next to the walls and bottom.
The first position in each of the legs, the positions next to the channel, are called channel positions. The channel position in each leg has the most glass passing over it than any of the remaining bushings in the leg, and the velocity of the molten glass passing over the channel positions can be significantly higher than it is further down the leg. When hot glass dives into the orifices feeding the channel positions, it substantially increases the break rate of the bushing and also increases the variation of the fiber diameters of the fiber coming from the bushing due to the higher temperature gradient this condition causes across the tip plate.
The use of a screen having a non-uniform hole size and/or hole density is taught by U.S. Pat. No. 4,612,027, but this reference does not suggest using that screen for addressing the above described problem. This patent teaches making a bushing having a dripless tip area and a dripping tip area in the tip plate of the bushing. The bushing taught by this patent has a screen that has much less resistance to flow in the center portion of the screen than the portion or area adjacent each end of the screen, i.e. the center portion of the screen has much larger holes and/or a higher hole density than the areas of the screen adjacent each end of the bushing. Also, the bushings taught by this patent must have vertical walls extending from the top of the tip plate to the bottom of the screen to separate the areas of different rates of molten glass flow to function as taught. Nothing in this patent discloses or suggests a solution to the problem of bad tip temperature profile in channel positions and the negative results this condition causes. Instead, this patent accepts that the tips on the outer periphery break out more frequently and teaches a bushing that tolerates such frequent fiber breaks and lower fiberizing efficiency.